The present invention relates to a method for the preparation of an unsaturated alcohol or, more particularly, to a method for the preparation of an ethylenically unsaturated alcohol useful as an intermediate for the synthetic preparation of various kinds of sex pheromone compounds of insects used for the population control of pests, insecticides, growth regulators of plants and insects and the like.
Various synthetic methods are known in the organic chemistry for the synthesis of unsaturated alcohols, of which the methods utilizing a Grignard reagent sometimes provide an efficient route for the synthesis of the desired compound. For example, a method is known in which a coupling reaction is performed between a Grignard reagent obtained by the reaction of magnesium with an .omega.-haloalcohol, i.e. halohydrin, having the hydroxy group protected with a protective group such as tetrapyranyl ether and an allyl compound such as allyl halides, allyl acetate, allyl methanesulfonate, allyl p-toluenesulfonate and the like.
The above mentioned coupling reaction of a Grignard reagent is not always quite satisfactory in respect of the low yeld of the desired compound and possible isomerization relative to the ethylenically unsaturated double bond which could be suppressed only by carrying out the reaction at a considerably low temperature at the sacrifice of the reaction velocity. In addition, the Grignard reagent per se is prepared by a troublesome process including the steps of protection of the hydroxy group in the starting .omega.-haloalcohol and deprotection, i.e. elimination of the protective group.
For example, Henrick in Tetrahedron, volume 33, page 1845 (1979) and Samain in Synthesis, page 388 (1978) disclosed a method for the synthesis of (E,E)-8,10-dodecadien-1-ol which is a sex pheromone compound of codling moths Laspeyresia pomonella in the reaction carried out at 0.degree. C. or below and expressed by the following reaction equation in the presence of an acid and Cu.sup.+ ions: EQU CH.sub.3 CH.dbd..sup.(E) CHCH.dbd..sup.(E) CHCH.sub.2 OCOCH.sub.3 +XMg(CH.sub.2).sub.6 O--Py .fwdarw.CH.sub.3 CH.dbd.CHCH.dbd.CH(CH.sub.2).sub.7 OH,
in which X is a halogen atom and Py is a 2-tetrahydropyranyl group.
Further, Descoin and Samain in Noueau Journal de Chimie, volume 2(3), page 249 (1978) disclosed a method for the synthesis of (E,Z)-7,9-dodecadienyl acetate which is a sex pheromone compound of European grapevine moths by the reaction carried out at -10.degree. C. or at -78.degree. C. and expressed by the following reaction equation in the presence of Li.sub.2 CuCl.sub.4 followed by hydrolysis and reaction with acetic anhydride: EQU CH.sub.3 CH.sub.2 CH.dbd..sup.(Z) CHCH.dbd..sup.(E) CHCH.sub.2 OCOCH.sub.3 +ClMg(CH.sub.2).sub.6 O--Py .fwdarw.CH.sub.3 CH.sub.2 CH.dbd..sup.(Z) CHCH.dbd..sup.(E) CH(CH.sub.2).sub.6 OCOCH.sub.3,
in which Py has the same meaning as defined above.
These synthetic methods are industrially not advantageous because the reaction must be carried out at a low temperature in a special reaction apparatus equipped with a cooling system. In addition, these methods are conducted in a troublesome process involving the steps of protection and deprotection of the hydroxy group. When the protective group of the hydroxy group is an organosilyl group, moreover, the Grignard reagent may sometimes be unstable as is shown by the following equation: EQU Cl(CH.sub.2).sub.n OSiR.sub.3 +Mg.fwdarw.[ClMg(CH.sub.2).sub.n OSiR.sub.3 ].fwdarw..fwdarw.R.sub.3 Si(CH.sub.2).sub.n OMgCl,
in which R is, for example, a methyl group and n is a positive integer. It is accordingly a conclusion that these prior art methods are not suitable as an industrial process for the mass production of an unsaturated alcohol. Therefore, it is eagerly desired to develop an industrially more advantageous method for the synthesis of unsaturated alcohols.